Flotation means for subsea well riser

ABSTRACT

Flotation is provided for a subsea well riser conduit. A plurality of open bottom but otherwise airtight buoyancy gas receiving chambers are mounted about the riser conduits. A gas conduit connects each chamber with a source of compressed gas on the drilling vessel. Valves are provided in the gas conduit for introducing gas in selectable amounts into the chambers to displace water entrained therein out the open bottom of the chamber to provide a selectable amount of buoyancy to the riser conduit. The gas valve includes an associated float for holding open the valve when the water level is above a predetermined level in the chamber and for closing the valve when the water level falls below the level to avoid loss of gas out the open bottom of the chamber. The flange which forms the top wall of the chamber is adapted to allow adjacent riser conduits to be fit together. Additionally, the chambers and the associated flanges provide a passageway for associated drilling control apparatus.

[lite atet [191 [111 $53,401 Watkins Jan. 7, 1975 FLOTATION MEANS FORSUBSEA WELL Primary Examiner]acob Shapiro RISER [57] ABSTRACT Flotationis provided for a subsea well riser conduit. A plurality of open bottombut otherwise airtight buoyancy gas receiving chambers are mounted aboutthe riser conduits. A gas conduit connects each chamber with a source ofcompressed gas on the drilling vessel. Valves are provided in the gasconduit for introducing gas in selectable amounts into the chambers todisplace water entrained therein out. the open bottom of the chamber toprovide a selectable amount of buoyancy to the riser conduit. The gasvalve includes an associated float for holding open the valve when thewater level is above a predetermined level in the chamber and forclosing the valve when the water level falls below the level to avoidloss of gas out the open bottom of the chamber. The flange which forms13 Claims, 6 Drawing; Figures Patented Jan. 7, 1975 3,858,401

2 Sheets-Sheet l Patented Jan. 7, 1975 I 3,858 401 2 Shasta-Sheet 2FLOTATION MEANS FOR SUBSEA WELL RISER BACKGROUND OF THE INVENTION Insubsea drilling, the long drilling riser conduit formed from conduitsections extends from the wellhead'to the vessel or floating platform.The riser conduit which is very heavy creates significant problems.

Wave action tends to move the vessel both vertically and horizontally.This is minimized by securely anchoring the vessel, but in deep waterdrilling, the vessel still undergoes rather large movement because ofslack and elasticity in the anchoring mechanism. If the vessel moves toofar horizontally, the elastic limit of the riser conduit may be reachedand it may break. Vertical oscillation of the vessel is usuallycompensated for by having a device which allows for relative movementbetween the riser conduit and the vessel. The vertical oscillation ofthe water also effects the upper portion of the riser conduit which mayresult in strain or ultimate breaking of the riser conduit. As thelength of the conduit and subsequently the weight thereof is increasedfor deeper drilling operations, these problems become more acute. Forexample, when the vessel moves horizontally and is no longer directlyover the wellhead, the riser conduit will not be directly vertical.Therefore, the weight of the upper sections of the riser conduit willtend to pull the vessel farther, horizontally, from its positiondirectly above the wellhead. As the vessel moves farther out and theconduit is farther from the vertical, the horizontal component of forcewill increase which will tend to drive the vessel even farther away.Clearly, if this were to continue, the elastic limit of the riserconduit could be reached and it could be deformed or break.

During storms or for routine maintenance, the vessel may have to leaveits associated conduit. It is necessary at that time to provide a forceto maintain the upper portion of the riser conduit near the surface ofthe body of water. 7

These problems associated with the riser conduit have been recognized,and it has been proposed to add flotation to a riser conduit to buoy it.Some early flotation systems utilized closed pressure vessels which wereattached to the riser conduit by attaching the top part of the buoyantvessel to the top part of the riser conduit section and the bottom partof the vessel to the bottom part of the riser conduit section. However,because of the temperature differential between the warm drilling mudinside the riser conduit and the cold sea water out side of it whichcause weakened sections of the riser conduit, the flotation method justdiscussed would cause non-uniform forces to act on the riser conduit.Some sealed buoyant vessels had to be extremely heavy to resist collapseand the high pressure in deep subsea wells, and they were extremelydifficult to handle. Third, the closed pressure vessels gave anon-adjustable amount of buoyancy which created problems in balancingthe flotation along the entire length of the riser conduit. It has alsobeen proposed to use foamed plastic around the riser conduit. However,these plastics are very expensive, and at great depth, they can absorbwater. Certain other improvements have been made in providing buoyancyfor a riser conduit. For example, see U.S. Pat. No. 3,572,041 to Graaf,dated Mar. 23, 1971, in which a plurality of buoyancy tanks are disposedinternally of a marine riser. These tanks are only placed in the topportion thereof so that the center of buoyancy is as high as possibleabove the center of gravity. In U.S. Pat. No. 3,667,240, dated June 6,1972, to Vilain, buoyancy is provided by a storage tank which is filledwith gas and hydrocarbons which creates a buoyant device. An externalbuoyant can arrangement is discussed in U.S. Pat. No. 3,720,066 toVilain dated Mar. 13,1973.

It has also been proposed to create chambers around the riser conduit toadd buoyancy thereto. See Rhodes, U.S. Pat. No. 3,017,934, dated Jan.23, 1962 in which buoyant cans which are opened at the bottom are filledfrom a supply of gas leading to the can nearest the ocean floor. A gasconduit allows the gas to flow from a full buoyant tank to the tankimmediately above it until all such tanks are filled. However, no gaswill flow to higher tanks until the bottom one is full. Therefore, thereis no adjustability in the system because no gas can enter an upper tankuntil the one directly below it is full. Also, in the aforementionedpatent the tanks are especially formed to fit around the riser conduit,and their sole function is buoyancy.

It is an object of the present invention to provide flotation for asubsea riser conduit which can be easily handled and would be morereliable then prior art flotation systems. It is a further object ofthis invention to have a low cost system and to have a system where almost any desired buoyancy may be provided. A further object of theinvention is to provide a buoyancy system which will also supportauxiliary lines and conduits associated with the riser conduit. It isalso an object of this invention to provide a device wherein amalfunction in part of the buoyancy system does not effect the rest ofthe buoyancy system. Another object of this invention is to provide abuoyancy device which will be of uniform size for each riser conduitsection used to ease handling and assembling the buoyancy means.

BRIEF SUMMARY OF THE INVENTION Flotation is provided for a subsea wellriser conduit run between a floating vessel provided with a source ofcompressed gas or air and a subsea wellhead. Such flotation meansincludes a plurality of open bottom buoyancy gas receiving chambers andmeans for mounting them about and along the riser conduits. Air or gasconduit means connected between the source of compressed gas and each ofthe chambers introduces gas in selectable amounts into the chamber,displacing water entrained therein out the open bottom of the chamber toprovide a selectable amount of buoyancy to the riser conduit. Gas valvemeans are provided at one or more of the chambers. The gas valve meansinclude associated float means for holding the valve means open when thewater level is above a predetermined level in the associated chamber andfor closing the valve when the water level falls below the level toavoid loss of gas out through the open bottom of the chambers. Arestricted orifice is provided between each chamber and the gas conduitmeans to provide a generally equal distribution of gas to each chamberfrom the gas conduit. The opened bottom buoyancy chamber includes anannular airtight flange formed integrally of and extending radiallyoutward from each of a plurality of riser conduit sections and aplurality of generally cylindrical airtight shells. Mounting means mountthe shells about the riser conduit in an airtight and dependingrelationship to an associated annular flange to thereby provide airtighttop and side walls to the open bottom chambers.

A centralizer is provided at the bottom of each chamher to generallymaintain the desired spacing between the shells and the conduitsections.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation depicting afloating platform or vessel over a subsea well site or formation withguide means interconnecting the vessel and the wellhead.

FIG. 2a is a sectional view taken along line II-II of FIG. 1 and showsthe bottom portion of one section of the riser conduit connected to thetop portion of another section of riser conduit.

FIG. 2b is a continuation of the bottom portion of the lower section ofriser conduit of FIG. 20 showing the connection between that section ofriser conduit and a third section of riser conduit.

FIG. 3 is a sectional view taken along line IIIIII in FIG. 2b.

FIG. 4 is a sectional view taken along line IVIV in FIG. 2b.

FIG. 5 is a detailed view of the gas conduit, the float, and theassociated valve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A subsea well riser orconductor conduit 20 extends from a floating vessel or platform throughthe body of water 12 to a wellhead 17. The vessel is suitably anchoredon the surface of the water. The subsea well riser conduit is run fromthe vessel or platform of slot 11, which is below the derrick 19, to thewellhead indicated generally at 17 which is mounted on the wellternplate 15 above the conductor pipe 14 in the formation 13.Conventional blowout preventer apparatus 16 and riser coupling apparatus18 may be additionally provided at the wellhead.

The subsea well riser conduit 20 is formed ofa plural ity of conductorconduit sections 21. Conductor conduit section 21a is the conduitsection nearest the vessel while conduit section 21d is nearest thewellhead. These conduit sections are generally approximately 40 to 50feet long. It is contemplated that the riser conduit of the instantinvention could be used in a drilling operation conducted at a depth of6,000 feet below the water surface.

Each conduit section has a cylindrical wall 22 having a top portion 23and a bottom portion 24. At the end of the top of the conduit section 23is a portion of larger inside diameter 25 which accepts the bottomportion of the conduit section mounted directly above it. O-rings 26seal the junction of the two adjacent conduit sections. An annularsection 27 is provided into which locking dogs clamp (FIG. 3) to holdthe adjacent conduit sections together. Bottom flange 28 and top flange29 project radially outward from the respective bottom and top portionof each conduit section. These flanges serve to guide the electrical orhydraulic lines 31 and 32, and a pipeline 51 carrying pressurized gaswhose function along with additional functions of the top flange will bediscussed hereinafter. In the preferred embodiment, tubing 31 and 32 arethe hydraulic control lines for the choke and the kill functions. Thesehydraulic lines are in sections of approximately the same length as ariser conduit section and are connected by connectors 33 and 34.Additionally, connector 34 functions to seal the opening in the topflange through which hydraulic line 31 passes.

flange 29 by mounting means 41 to form an airtight seal therebetween.Preferably, the mounting means includes a plurality of bolts around theperiphery of flange 29 connecting the flange to the annular shell 42.Centralizer ring 43 mounted near the bottom of the shell services themaintain the annular shell a fixed distance from the cylindrical wall ofthe riser conduit. The contemplated centralizer is a ring 45 with radialfins 46 extending from the outer surface of the riser conduit to theinside of the shell. To add more support to the shell, othercentralizers 47 may be spaced along each riser conduit section. There isno'seal to the bottom of the chamber and water is free to rise insidethe chamber.

The chambers run almost the total length of each riser conduit sectionand all are substantially the same size. This aids in handling them.Because the shells are of uniform dimensions throughout their length,the cost per shell is decreased and handling is facilitated.

Air or gas supply means, including a gas line or gas conduit means, isconnected between a source of compressed air or gas on the vessel andeach chamber for introducing gas in selectable amounts into the chamberdisplacing water entrained therein out the open bottom of the chamber toprovide a selectable amount of buoyancy to the riser conduit. In thepreferred embodiment such gas supply means 50 includes a gas line or gasconduit means 51 and gas valve means 52. The gas valve means includes anassociated float means for holding the valve means open when the waterlevel is above a predetermined level in the associated chamber and forclosing the valve means when the water level falls below the level toavoid loss of gas out through the open bottom of any chamber. Floatmeans 55 is connected by a stem or connector members 56 to a valvemember 57 above the valve seat 54 so that as the level of water 12 riseswithin the chamber 40, float means 55 causes the valve member 57 to moveupward from valve seat 55 in FIG. 5 and allow gas into the chamber. Thegas will displace water 12 inside the chamber until the level of waterreaches a low enough level so that float 55 is no longer supported onthe water and drops a sufficient distance to close the valve seat 54 ofvalve 52. Therefore, even though there is more water pressure on thelowermost riser conduit sections than on the uppermost riser conduitsections, when the upper riser conduit sections fill with gas, the valvewill then close and gas can flow into the chambers associated with thelower conduit sections instead of having gas leaking from the bottoms ofthe chambers associated with the upper conduit sections.

Additionally, a means for restricting gas flow is provided between eachchamber and the gas conduit means for providing a generally. equaldistribution of gas from the gas conduit to the chambers. In thepreferred embodiment, such means includes the provision of a restrictedpassage 53 formed between stem 56 and the orifice in valve seat 54. Thegas flow restricting chamber to balance the flow of air between therespective chambers.

The gas conduit means passes through an aperture in the top flange 29.This aperture is sealed by the seal means 59. It should be noted thatseals 57 and 34 and the means 41 mounting the shell 42 to the top flange29 must cause an airtight top portion because the gas pressure withineach chamber, especially those farther beneath the surface of the waterwill be extremely high. In addition, spacer element 58 is provided tofit against the bottom flange.

By severly limiting the capacity of the valve or by the use of a verysmall orifice and by using very high pressure gas in the conduit, thefailure of a valve to close in one or two chambers would not cause allchambers to cease filling with gas. Some gas would simply be dischargedat the bottom of the chambers with a valve failure until the otherchambers were filled with gas. The overall gas injection rate would notbe reduced because many chambers would normally be fillingsimultaneously even through the chambers nearer the surface fill fasterbecause the water pressure is lower there.

The failure of a single valveto close after all the chambers are filledis also not a significant problem. If the source of compressed gas is nolonger delivering gas to the chamber, the water level will rise as gasflows back through the valve. However, the gas above the water isnecessarily at the same pressure as the water. Therefore, the waterlevel cannot rise above the valve. If flotation is to be completelyadjustable (buoyancy addable or subtractable) a bleed line or other gasremoving means (not shown).should be provided near the top of eachchamber.

It should be recognized also that if each chamber is partially filledwith gas to allow for a certain amount of buoyancy, the chamber willhold that amount of gas and maintain that amount of buoyancy.

When all chambers are full, there will be less buoyancy for the chambersnearer the wellhead because the 3 5 gas in those chambers is underhigher pressure and therefore is heavier per unit volumn. There will beno tendency for a more buoyant lower portion of the riser conduit tofloat to a higher position than the upper por' tion of the riserconduit. The slightly more buoyant upper chambers have a tendency tomaintain the conduit vertically. This limits horizontal movement.

As the buoyancy to the system is increased, the riser can becomefree-standing to allow the vessel to leave its associated conduit. Thetop portion should then terminate a sufficient distance below the waterlevel to not become a hazard to navigation. A marker buoy would beattached to the riser to facilitate in relocating it.

Buoyancy means for use with subsea well apparatus have been shown whichincludes a marine conductor conduit 20 run from a floating vessel to asubsea well and which is formed of conduit sections 21 connectedtogether in an end-to-end relationship. The buoyancy means comprisesflange means 29 extending radially outwardly from and about one or moreof the plurality of the conduit sections in an airtight relationshipthereto. There are a plurality of airtight shells 42 and mounting means41 for mounting the shells positioned about the one or more of theconduit sections and extending downwardly from the associated flangemeans to form a plurality of buoyancy chambers which are open at theirbottoms. Gas supply means supply selectable amounts of gas from a sourceon the vessel to each chamber, the introduction of gas into the chambersdisplacing water therein out through the open bottom of the chambers.

I claim:

l. Flotation means for a subsea well riser conduit run between afloating vessel provided with a source of compressed gas and a subseawellhead, said flotation means comprising:

a plurality of open bottomed buoyancy gas receiving chambers and meansfor mounting them about and along said riser conduit; and

gas supply means including gas conduit means connected between saidsource of compressed gas and each of said chambers for introducing gasin selectable amounts into each of said chambers independently of theothers, displacing water entrained therein out open bottoms of saidchambers, to pro vide selectable amounts of buoyancy to said riserconduit.

2. The flotationmeans of claim 1 wherein gas valve means are provided atone or more of said chambers, each of said gas valve means including avalve member and float means mechanically connected thereto for 20holding said valve means open when the water level is above apredetermined level in the associate chamber and for closing said valvemeans when the water level falls below said level'to avoid loss of gasout through said open bottom of said chambers.

3. The flotation means of claim 1 wherein means for restricting 'gasflow is-provided between each chamber and the gas conduit means toprovide for a generally equal and simultaneous distribution of gas fromsaid gas conduit to said chambers.

4. The flotation means of claim 1 wherein one or more of said pluralityof open bottomed buoyancy chambers comprises: i

an annular flange formed integrally of and extending radially outwardlyof each of a plurality of riser conduit sections;

a plurality of generally cylindrical shells; and

mounting means for mounting said shells about said riser conduit inairtight and depending relation to an associated annular flange tothereby provide airtight top and side walls of the open bottomedchambers.

5. The flotation means of claim 4 further including aperture meansthrough said annular flange for passing said gas conduit meanstherethrough.

6. The flotation means of claim 1 wherein said gas conduit extends intoone or more of said chambers through the upper ends of said chamberscloser to the floating vessel, and gas in said conduit flows in adirection from the source of compressed gas through said upper ends ofsaid chambers.

7. The flotation means of claim 6 wherein gas valve means are providedin one ormore of said chambers near said open bottom of said chambers toallow gas to 55 pass only from said gas conduit into said chambers.

8. Buoyancy means for use with subsea well apparatus which includes amarine conductor conduit run from floating vessel to subsea well andwhich is formed of conduit sections connected together in an end-to-endrelationship, said buoyancy means comprismg:

flange means extending radially outwardly from and about one or more ofsaid plurality of conduit sections in airtight relation thereto;

a plurality of airtight shells and mounting means for mounting saidshells positioned about said one or more of said sections and extendingdownwardly from the associated flange means to form a plurality ofannular buoyancy chambers which are open at their bottoms; and 7 gassupply means for supplying selectable amounts of gas from a source onsaid vessel to each of said chambers; introduction of gas into saidchambers displacing water therein out through said open bottoms of saidchambers.

9. The buoyancy means of claim 8 wherein said gas supply means includesa gas line run from said vessel to said chambers and gas valve meanswithin one or more of said chambers.

10. The buoyancy means of claim 9 wherein each of said gas valve meansincludes means for restricting gas flow associated with its gas line toregulate the rate of gas flow between the gas line and respectivechamber to provide for a substantially equal pressurizing of saidchambers.

11. The buoyancy means of claim 9 wherein each of said gas valve meanscomprises a valve seat and associated valve member for blocking the flowof gas through the valve means when said member is seated on said valveseat and float means connected to said valve member to open said valvemeans when said float is buoyed by surrounding water in said chamber,said valve means closing to stop the flow of gas into said chamber whenthe water level within said chamber recedes under the pressure of gasintroduced therein so that said float no longer supports said valvemember above said valve seat.

12. The method of providing buoyancy to a marine conductor conduit runfrom a floating vessel to a subsea well comprising the steps of:

providing a plurality of gas receiving buoyancy chambers with openingsat their bottom ends in a vertical array along said marine conductorconduit; and

after running of said conductor conduit to said well from said floatingvessel, introducing gas from a source thereof on said vessel into eachof said chambers independently of the others and forcing water entrainedtherein out through the openings at the chamber bottoms to provide aselectable amount of buoyancy therefor.

13. The method of claim 12 including the additional steps of:

providing water level activated gas valves in said chambers andintroducing said gas through said valves, and

closing said valves by the lowering of the water level in said chambersduring said step of forcing water out of said chambers.

1. Flotation means for a subsea well riser conduit run between afloating vessel provided with a source of compressed gas and a subseawellhead, said flotation means comprising: a plurality of open bottomedbuoyancy gas receiving chambers and means for mounting them about andalong said riser conduit; and gas supply means including gas conduitmeans connected between said source of compressed gas and each of saidchambers for introducing gas in selectable amounts into each of saidchambers independently of the others, displacing water entrained thereinout open bottoms of said chambers, to provide selectable amounts ofbuoyancy to said riser conduit.
 2. The flotation means of claim 1wherein gas valve means are provided at one or more of said chambers,each of said gas valve means including a valve member and float meansmechanically connected thereto for holding said valve means open whenthe water level is above a predetermined level in the associate chamberand for closing said valve means when the water level falls below saidlevel to avoid loss of gas out through said open bottom of saidchambers.
 3. The flotation means of claim 1 wherein Means forrestricting gas flow is provided between each chamber and the gasconduit means to provide for a generally equal and simultaneousdistribution of gas from said gas conduit to said chambers.
 4. Theflotation means of claim 1 wherein one or more of said plurality of openbottomed buoyancy chambers comprises: an annular flange formedintegrally of and extending radially outwardly of each of a plurality ofriser conduit sections; a plurality of generally cylindrical shells; andmounting means for mounting said shells about said riser conduit inairtight and depending relation to an associated annular flange tothereby provide airtight top and side walls of the open bottomedchambers.
 5. The flotation means of claim 4 further including aperturemeans through said annular flange for passing said gas conduit meanstherethrough.
 6. The flotation means of claim 1 wherein said gas conduitextends into one or more of said chambers through the upper ends of saidchambers closer to the floating vessel, and gas in said conduit flows ina direction from the source of compressed gas through said upper ends ofsaid chambers.
 7. The flotation means of claim 6 wherein gas valve meansare provided in one or more of said chambers near said open bottom ofsaid chambers to allow gas to pass only from said gas conduit into saidchambers.
 8. Buoyancy means for use with subsea well apparatus whichincludes a marine conductor conduit run from a floating vessel to asubsea well and which is formed of conduit sections connected togetherin an end-to-end relationship, said buoyancy means comprising: flangemeans extending radially outwardly from and about one or more of saidplurality of conduit sections in airtight relation thereto; a pluralityof airtight shells and mounting means for mounting said shellspositioned about said one or more of said sections and extendingdownwardly from the associated flange means to form a plurality ofannular buoyancy chambers which are open at their bottoms; and gassupply means for supplying selectable amounts of gas from a source onsaid vessel to each of said chambers; introduction of gas into saidchambers displacing water therein out through said open bottoms of saidchambers.
 9. The buoyancy means of claim 8 wherein said gas supply meansincludes a gas line run from said vessel to said chambers and gas valvemeans within one or more of said chambers.
 10. The buoyancy means ofclaim 9 wherein each of said gas valve means includes means forrestricting gas flow associated with its gas line to regulate the rateof gas flow between the gas line and respective chamber to provide for asubstantially equal pressurizing of said chambers.
 11. The buoyancymeans of claim 9 wherein each of said gas valve means comprises a valveseat and associated valve member for blocking the flow of gas throughthe valve means when said member is seated on said valve seat and floatmeans connected to said valve member to open said valve means when saidfloat is buoyed by surrounding water in said chamber, said valve meansclosing to stop the flow of gas into said chamber when the water levelwithin said chamber recedes under the pressure of gas introduced thereinso that said float no longer supports said valve member above said valveseat.
 12. The method of providing buoyancy to a marine conductor conduitrun from a floating vessel to a subsea well comprising the steps of:providing a plurality of gas receiving buoyancy chambers with openingsat their bottom ends in a vertical array along said marine conductorconduit; and after running of said conductor conduit to said well fromsaid floating vessel, introducing gas from a source thereof on saidvessel into each of said chambers independently of the others andforcing water entrained therein out through the openings at the chamberbottoms to provide a selectable amount of buoyancy therefor.
 13. Themethod of claim 12 including the additional steps of: providing waterlevel activated gas valves in said chambers and introducing said gasthrough said valves, and closing said valves by the lowering of thewater level in said chambers during said step of forcing water out ofsaid chambers.